The invention relates generally to direct-acting pressure gauges, and more particularly to a pressure sensor which is operable in the low-pressure range and which includes means for adjusting the range thereof.
Measurement of absolute pressure, gauge pressure, vacuum and draft pressure, and differential pressure, is carried out by two primary types of pressure-sensitive elements, the first being the liquid column in which the height and density of the liquid are utilized to measure pressure, and the second being the elastic pressure device. The sole concern of the present invention is with elastic pressure elements which are designed to follow the physical law that within an elastic limit, stress is proportional to strain; hence deflection is proportional to applied pressure.
The Bourdon tube, because of its stability, simplicity and high pointer torque, is widely used as a pressure or vacuum indicator. The operation of the Bourdon tube is based on the principle that an elastic tube having an internal cross-section that is not a perfect circle, if bent or distorted, has the property of changing its shape with internal pressure variations. This internal pressure causes the cross-sectional form to become more circular, giving rise to a motion of the closed end or tip of the tube if the open end is rigidly, fixed, this motion being usually referred to as "tip travel."
While Bourdon tubes are capable of operating within various pressure ranges running as high as 100,000 psi, in no instance is it possible as a practical matter, to operate such tubes below 12 or 15 psi. Hence, despite the advantages of Bourdon tubes, they are not effective as gauges in the low-pressure range and it has heretofore been necessary to employ costly and relatively fragile bellows or diaphragm-sensing elements to effect measurement in this range.
My prior U.S. Pat. No. 3,732,733 discloses a low-cost pressure sensor capable of accurately gauging pressures in the low pressure range. The sensor disclosed in this patent includes two curved flat metal springs in a concentric arrangement wherein the springs are cantilevered from a socket, the free end of the springs being joined together at a tip from which a pointer extends. The springs enclose a flexible bladder having an internal chamber which communicates with the socket, whereby fluid fed through the socket into this chamber acts to expand the bladder and causes the springs to uncurl, thereby moving the tip and the pointer attached thereto as a function of the applied pressure.
In a conventional Bourdon tube pressure gauge, the relationship between applied fluid pressure and tip travel is such that it takes a large pressure change to effect a relatively small tip movement. Consequently, commercial forms of such gauges include gear works or linkages to mechanically amplify the tip travel. The friction introduced by mechanical amplifiers usually makes it necessary to tap the gauge to cause it to register. Such tapping is unnecessary with a sensor of the type disclosed in my prior patent, for no mechanical amplifier is involved. The sensor is highly sensitive so that a small change in applied pressure produces a relatively large tip excursion.
Moreover, with this patented sensor, since there are no gears or linkages and the tip is directly coupled to the pointer, there is no problem of wear and the sensor has a high shock resistance as well as a long life expectancy.
In a commercial embodiment of a pressure sensor of the type covered by the above-noted patent, the pointer extending from the tip describes a true arc along a curved scale. The pointer motion is linear and can be adjusted by changing the active length of the two springs. This is done by means of factory calibration procedure in which a clip is attached at a selected position which determines the active length of the springs. By this expedient, it is possible to adjust the range of the sensor approximately 25%.
This factory testing and calibration procedure for adjusting the range of the sensor adds substantially to the cost of production. Moreover, once the sensor is enclosed in its case, range adjustment is no longer possible and should an adjustment be necessary in the field, this cannot be done.
Another drawback of the sensor disclosed in the above-noted patent is that since its bladder was unattached to the springs between which it was sandwiched, the sensor could not function as a negative pressure or vacuum gauge, for negative pressure would cause the bladder to collapse.